The interrelationships between calcium and cyclic nucleotides in the control of platelet secretion and aggregation will be studied. The effects of cyclic AMP, cyclic GMP and protein kinases on calcium transport by a platelet microsomal fraction will be investigated. In addition the effects of calcium on adenylate and guanylate cyclases and phosphodiesterases will be explored. Intracellular localization of enzymes involved in calcium and cyclic nucleotide metabolism will be investigated by cytochemical electron microscopic methods. The effects of aggregating agents, and inhibitors of aggregation will be studied with respect to the intracellular distribution of calcium.